The present invention relates to electrocardiology and in particular to an arrhythmic analyzing system for electrocardiographs and more particularly to such a system for digitally scanning and characterizing arrhythmic electrocardial signals during high speed playback of ECG recordings.
Electrical signals that appear at the surface of a subject's skin as a result of the electrical activity within the heart are known as ECG signals. These ECG signals exhibit particular wave forms which correspond to the action within the heart muscle and reflect both in timing and in character the condition of the heart. It is well known to place electrodes on the patient's skin to sense the ECG signals and to present them for visual analysis either in real time or at a subsequent time for use by a physician or other trained personnel. Of interest to the physician is an analysis over an extended period of time of a continuous ECG signal which reflects heart activity during the normal activities of the patient and which displays anomalous heart signals which represent arrhythmic and ectopic activity. As used herein arrhythmic heart activity includes activity which is abnormal, such as irregular variations in rhythm and ectopic activity such as other arrhythmia, ventricular ectopic beats and abberently conducted supraventricular beats. More specifically, it is desired to accumulate and analyze large volumes of ECG signals during normal activities of the patient as for example over a twenty four hour period and to review this accumulation in a highly accelerated mode to determine the frequency and character of the various heart signals. For this purpose, there are known systems which accomplish the foregoing by recording the ECG signals in real time on a small, compact tape recorder which is worn by the patient. The recorded signals are then processed by replaying the same at a much faster speed with a presentation of the signals on a cathode ray oscilloscope in which each ECG complex is superimposed on predecessor complexes. This type of display is known as a Holter display, and a description of such a system is set forth in U.S. Pat. No. 3,215,136 issued Nov. 2, 1965 in the name of Norman J. Holter, et al. The scanning device of U.S. Pat. No. 3,215,136 utilizes superimposition of the ECG signals from two different tracks of the magnetic recording tape. A subsequent improvement is set forth in U.S. Pat. No. 3,718,772 issued Feb. 27, 1963 in the name of Clifford Sanctuary.
U.S. Pat. No. 4,157,571 issued June 5, 1979 to Steven K. Shu discloses an improved scanner in which the tape is reviewed by the Holter technique at high speed. The operator is also able to stop the high speed Holter display and to initiate a real-time single-frame viewing mode in which a segment of tape is repeated, any interval of the signal representing a frame stored in digital memory, and the content of memory displayed in real time for analysis by the operator and selected printout as desired.
Prior art scanners have also included provision for analyzing the signal to detect various abnormalities.
Normally the ECG signal results from electrical impulses that are generated to initiate the polarization of the ventricles of the heart at contraction and this signal is commonly referred to as a QRS complex. Immediately preceding this complex there is a small pulse that represents the initiation of the muscular activity and is referred to as a P-wave. Following the QRS complex there is at least one additional pulse commonly referred to as a T-wave. It is separated from the QRS wave by the so-called ST segment. Following the completion of each pumping action and prior to the succeeding pumping action, the heart relaxes and the ECG signal is essentially quiet with little or no fluctuation in electrical output.
In electrical pulses from a normal heart, the rhythm and the shape of the QRS complex are found to have certain predetermined characteristics which fall within certain limits. However, in the event the heart is subjected to abnormal strain or has infirmities, the ECG signal may exhibit more differences from a normal ECG signal. For example, the rate and rhythm of the heart beat may be erratic and may vary throughout wide limits. In addition the size, shape, and duration of the QRS complex and ST segment following that may substantially vary from the normal. These are generally termed ectopic beats, the most important being ventricular ectopic (VE) and supraventricular extopic (SVE). In general the cardiologist needs to know the number of VE and SVE beats in a given time interval.
U.S. Pat. No. 3,267,934 dated Aug. 23, 1966 to William E. Thornton entitled Electrocardiac Computer provides a means responsive to one or more of the characteristics of each QRS signal to measure its value and to detect the occurrence of ectopic beats, ST segment depression, pulse rate arrhythmia and other characteristics. U.S. Pat. No. 3,267,933 to W. E. Mills et al., dated Aug. 23, 1966, U.S. Pat. No. 3,858,034 to Donald L. Anderson dated Dec. 31, 1974, U.S. Pat. No. 4,006,737 to Isaac R. Cherry dated Feb. 8, 1977, and U.S. Pat. No. 4,073,011 to Isaac R. Cherry et al. dated Feb. 7, 1978 all disclose various computer-assisted analysis systems for processing cardiographic signals derived from high speed scanning of ECG tapes. These systems provide for identification of normal ventricular ectopic (VE), supraventricular ectopic (SVE), and other abnormalities and for accumulating a readout totalling such events over a scanning period. To do this, provision was made for establishing a set of signal threshold conditions against which each complex was compared, specifically with respect to pulse rate prematurity of the R--R intervals, R-wave, width, and R-wave amplitude. In general, such systems are either preset, i.e., contain a single set of built-in measures that determine excessive QRS width, amplitude, or other characteristics which are used to identify ectopic beats, or provide for operator analysis and selection from among a single set of predetermined characteristics for characterizing ectopic beats. It has been found, however, that it is difficult, if not impossible, to adequately establish a single set of predetermined criteria for normal, VE, and SVE beats which can be built into the analyzing circuitry of the scanner for satisfactory arrhythmic analysis, for it is desired to take into account the difference between patients in that normal VE and SVE beats vary widely from patient ot patient and can also vary with the particular ECG hookup or the position of the torso. Referring particularly to U.S. Pat. No. 4,076,011, the arrhythmia computer there provided for the automatic detection of an unusual event within the ECG signal such as the occurrence of ectopic beats. Provision was made for the operator to select whether or not to use present criteria for prematurity, width, amplitude and to select a single width value limiting normal events. The recommended procedure for selecting appropriate ectopic criteria called for the operator to scan a portion of the patient tape containing ectopic activity and to obtain a sample writeout. The writeout was then measured and used to determine which of the selectable criteria of the arrythmic computer were to be activated for counting ectopic occurrences on each individual patient tape recording. After the criteria were selected the entire patient tape was then rescanned to provide a complete analysis and digital display of totalled VE and SVE beats. While that system provided valuable assistance in the analysis of Holter-type recordings, it suffered from certain disadvantages and limitations, specifically the criteria for selection and classification of ectopic beats were built into the system and, once selected, became a single set of nonadjustable criteria. Additionally, whether or not each of the criteria should be employed required an operator choice and physical measurement of selected writeouts after reviewing the considerable portion of ectopic activity. Also, the variations in characteristics between abnormal beats exhibited within a given patient are often such that the criteria set for one abnormal beat will not permit another different beat to be detected. Thus, such prior art systems did not allow for adequate variation in selection of the various criteria by which VE and SVE activity were to be measured to accommodate patient differences and provided no breadth or range in the criteria used, once those criteria were selected. There is, therefore, a need for a new and improved ECG analysis system.